[clang] Implement `__is_pointer_interconvertible_base_of()` (#88473)

This patch implements intrinsic that supports
`std::is_pointer_interconvertible_base_of` type trait from
[P0466R5](https://wg21.link/p0466r5) "Layout-compatibility and
Pointer-interconvertibility Traits".

Normative wording:
> Comment: If `Base` and Derived are non-union class types and are not
(possibly _cv_-qualified) versions of the same type, `Derived` is a
complete type.
> Condition: `Derived` is unambiguously derived from `Base` without
regard to _cv_-qualifiers, and each object of type `Derived` is
pointer-interconvertible (6.7.2 [basic.compound]) with its `Base`
subobject, or `Base` and `Derived` are not unions and name the same
class type without regard to _cv_-qualifiers.

The paper also express the following intent:
> Note that `is_pointer_interconvertible_base_of_v<T,T>` is always true
under this wording, even though `T` is not derived from itself.

I find the treatment of unions in the wording contradictory to this
intent, and I'm not able to find anything relevant in minutes or on the
reflector. That said, this patch implements what the wording says, since
it's very explicit about unions.